Since devices of this type cannot be moved beyond a certain terminal position, the associated d-c motor tends to become overloaded and must therefore be protected against excessive current flow in its armature winding which would not only drain the vehicular battery but also cause overheating with possible destructive effects. Conventional protective systems used for this purpose can be divided into two groups, namely those operating mechanically to cut off the motor in response to a physical obstacle encountered by the load and those using the armature current as a cut-off criterion.
Typical overload-protection systems of the first group, comprising spring-loaded disconnect switches, are described in commonly owned German Pat. No. 1,079,133 and German published specification No. 25 12 791. Also included in this group are systems using a slipping clutch, e.g. as described in commonly owned German utility model No. 7,125,992.
A protective system of the second group, described in commonly owned German published specification No. 2 001 745, comprises a resistor in series with the armature winding and means for de-energizing this winding upon detecting an excessive voltage drop across the series resistor due to a stoppage of the load (i.e. a telescoping radio antenna) in a terminal position. This system also includes a delay circuit designed to prevent premature de-energization during start-up when the armature winding draws a large current.
An electronically operating motor-control system of the kind last described avoids the disadvantages of bulk, wear and lack of convenient adjustability inherent in protective systems of the first group, yet the need for letting the armature current rise--albeit for a brief period--above a normally admissible limit is still a drawback, requiring the rotor and its winding to be designed in an uneconomical manner for larger loads than would basically be necessary